You've got until June to get Windows 10 for free. After that the upgrade will cost $120.
I just found out that the date is in July, thanks to a member on another forum I'm on.
http://windows.microsoft.com/en-us/windows-10/upgrade-to-windows-10-faq
What is the free upgrade to Windows 10?
Microsoft is making Windows 10 available as a free upgrade for compatible devices that are running genuine Windows 7 Service Pack 1 or Windows 8.1 Update.
The free upgrade is a full version of Windows (not a trial or introductory version) and is available until July 29, 2016. Once you upgrade, you’ll have Windows 10 for free on that device.